Nutrition when studying?

Msdwyer17

I am studying for my a levels, which start in 2 weeks and then last 2 weeks.
I recently found out I have pcos and am insulin resistant and should cut down on sugar. I want to cut down on carbs and do this however whenever I try I feel like I have little energy and bad moods, which interfere with my study.

I also keep getting told carbs and sugar fuel the brain and are good for exams, help what do I do? Change my diet now or after exams?
I feel like I'm crashing from sugar a lot and can't concentrate but find that without it the withdrawal is too much!!